Currently, in course of the manufacturing of semiconductor products, the manufacturing process of a product often includes a dozen of and even dozens of photolithography processes so as to form various circuit patterns and the like. In general, except for the first photolithography process, in the subsequent remaining photolithography processes, patterns of the current level must be aligned with patterns of the previous level before exposure.
Description will be given below by taking the exposure process of a thin-film transistor (TFT) array substrate in a liquid crystal display (LCD) device as an example. The TFT array substrate comprises a substrate and a plurality of film layers formed on the substrate. In the photolithography processes of the plurality of film layers, the traditional alignment method adopted in an exposure process is as follows. Firstly, a plurality of groups of alignment marks are formed in alignment areas A of a first film layer directly formed on the substrate, and are used as a reference scale for the pattern alignment between the subsequent film layer and the first film layer. Secondly, when the alignment and exposure process is conducted for a subsequent film layer, a group of the alignment marks 1 as shown in FIG. 1 are alignment marks formed in the first film layer and used for a mask plate for the subsequent film layer. For instance, when the alignment and exposure process is conducted for the second film layer on the first film layer, a group of alignment marks 2 used by a mask plate for the second film layer are aligned with the group of the alignment marks 1 in the first film layer, so that patterns formed by the second film layer and patterns formed by the first film layer can be accurately aligned. The alignment marks 2 in the mask plate for the second film layer are as shown in FIG. 2.
After the alignment process is conducted in accordance with the above alignment method, the second film layer is subjected to exposure and development; patterns corresponding to the marks 2 will be formed, due to the marks 2 of the mask plate used for the second film layer, at corresponding positions of the marks 1; and hence the patterns in the alignment areas A, formed by the first film layer, are converted into combined patterns of the marks 1 and the marks 2, as illustrated in FIG. 3. Therefore, after the photolithography process for the second film layer is completed, a further subsequent film layer formed on the second film layer cannot use the marks 1 for alignment again in the photolithography process but needs to use a group of marks at other positions for alignment. In this case, each film layer after the first film layer has to use a different group of marks in the first film layer for alignment, so that the alignment offset amounts and the alignment offset directions among various film layers are inconsistent on another, and hence the alignment error between patterns formed after the exposure for the various layers become larger, and consequently the number of unqualified products of the TFT array substrate can be increased.